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I searched for this but couldn't find the answer i was looking for. I have a 335i coupe with ZSP. I checked the pressure in my tires and they were ~42psi front ~45psi rear. I put it down to the suggested pressure on the door but after taking some corners, i felt that the car understeered a lot more and felt sluggish. I put it up to 40 front and 42 rear and it feels kinda hard now. What is the ideal pressure to have so that it wont be too hard or too soft.

Optimal tire pressures are a bit subjective and each driver will have their own 'best.' You are really going to have to keep doing what you just mentioned (experimenting) to find your personal preference.

However, remember there are 2 different pressures that BMW recommends, one set for normal load/driving and 1 for speeds over 100 mph. I have found that I prefer 35F / 37R which provides me with the best steering response and ride comfort while keeping understeer relatively low. This rear pressure is a little below BMWs recommended pressure, so I do have to up it a little bit if I am going to be hauling 2 people/luggage in the back.

If you are experiencing some understeer try upping the fronts closer to whatever your rears are set at.

40F and 42R is a bit high, especially for RFTs which already have a very stiff sidewall. Try starting with 35F/37R and play with it from there.

I forgot to mention that when i lowered the pressure down to what was recommended on the door, i got a low tire pressure signal. And one more question.. For speeds above 100, is it better if the tires have more pressure or less?

I forgot to mention that when i lowered the pressure down to what was recommended on the door, i got a low tire pressure signal. And one more question.. For speeds above 100, is it better if the tires have more pressure or less?

No problem. The link in the post above can explain all of this much better than I can.

Have fun experimenting. My neighbors make fun of me because they see me out there in the mornings fooling with my air pressure (just installed new Michelin PS2 runflats so still experimenting for myself on these tires). Of course, they drive honda civics with touring tires, so I guess they just don't understand.

I forgot to mention that when i lowered the pressure down to what was recommended on the door, i got a low tire pressure signal. And one more question.. For speeds above 100, is it better if the tires have more pressure or less?

If you change the pressures you have to reset the pressure system.
Also remember that pressure is a function of temperature. So if you measured the pressures after you have been driving for a while, the tires will be warm and the pressure will be higher. Usually the recommended pressures are cold tire pressures.